Add support for C99

Author: nradakovic

docs/features.md

@@ -27,12 +27,20 @@ enabled by default.
 - **`supports_header_path_normalization`** (Linux) — Suppresses absolute-path
   warnings for system headers.
 - **`dbg`** / **`opt`** (both) — Well-known build-mode markers used to select flags.
+- **`gnu11`** (Linux) *opt-in* — Switches C compilation from the default
+  `-std=c11` to `-std=gnu11`.
+- **`c99`** (both) *opt-in* — Switches C compilation from the default
+  `-std=c11` to `-std=c99`.
 
 ## Compilation
 - **`unfiltered_compile_flags`** (both) — Redacts `__DATE__`/`__TIME__`/`__TIMESTAMP__`
   for reproducible builds.
 - **`default_compile_flags`** (both) — Core compile flags plus `dbg`/`opt`
-  build-mode variants (exact flags differ per target).
+  build-mode variants (exact flags differ per target). C compilation defaults
+  to `-std=c11`; request `--features=gnu11` (Linux) or `--features=c99` (both)
+  to switch the C standard. Only one of `c11`/`gnu11`/`c99` should be active
+  at a time — requesting `gnu11` and `c99` together applies both `-std=` flags,
+  with the compiler honoring the last one seen.
 - **`pic`** (Linux) — Emits `-fPIC` when the `pic` build variable is available.
   QNX declares the `supports_pic` capability marker but does not add `-fPIC` in
   the default compile flags.

features/custom/linux/make_cc_features.bzl

@@ -17,6 +17,7 @@ _LINUX_FEATURES = [
     ("@score_bazel_cpp_toolchains//features/native/markers:dbg", False),  # Bazel auto-toggles via -c dbg
     ("@score_bazel_cpp_toolchains//features/native/unfiltered_compile_flags", True),
     ("@score_bazel_cpp_toolchains//features/native/markers:gnu11", False),  # opt-in
+    ("@score_bazel_cpp_toolchains//features/native/markers:c99", False),  # opt-in
     ("@score_bazel_cpp_toolchains//features/native/default_compile_flags", True),
     ("@score_bazel_cpp_toolchains//features/native/random_seed", True),
     ("@score_bazel_cpp_toolchains//features/native/include_paths", True),

features/custom/qnx/make_cc_features.bzl

@@ -38,6 +38,7 @@ _QNX_FEATURES = [
     ("@score_bazel_cpp_toolchains//features/native/markers:no_legacy_features", True),
     ("@score_bazel_cpp_toolchains//features/native/unfiltered_compile_flags", True),
     ("@score_bazel_cpp_toolchains//features/custom/qnx/gcc_version_flags", True),
+    ("@score_bazel_cpp_toolchains//features/native/markers:c99", False),  # opt-in
     ("@score_bazel_cpp_toolchains//features/native/default_compile_flags", True),
     ("@score_bazel_cpp_toolchains//features/native/random_seed", True),
     ("@score_bazel_cpp_toolchains//features/native/include_paths", True),

features/native/default_compile_flags/BUILD

@@ -53,14 +53,22 @@ cc_args(
 
 cc_feature_constraint(
     name = "not_gnu11",
-    none_of = ["//features/native/markers:gnu11"],
+    none_of = [
+        "//features/native/markers:gnu11",
+        "//features/native/markers:c99",
+    ],
 )
 
 cc_feature_constraint(
     name = "is_gnu11",
     all_of = ["//features/native/markers:gnu11"],
 )
 
+cc_feature_constraint(
+    name = "is_c99",
+    all_of = ["//features/native/markers:c99"],
+)
+
 cc_args(
     name = "c_std_c11_args",
     actions = ["@rules_cc//cc/toolchains/actions:c_compile_actions"],
@@ -75,6 +83,13 @@ cc_args(
     requires_any_of = [":is_gnu11"],
 )
 
+cc_args(
+    name = "c_std_c99_args",
+    actions = ["@rules_cc//cc/toolchains/actions:c_compile_actions"],
+    args = ["-std=c99"],
+    requires_any_of = [":is_c99"],
+)
+
 cc_args(
     name = "default_cxx_compile_flags_args",
     actions = ["@rules_cc//cc/toolchains/actions:cpp_compile_actions"],
@@ -152,6 +167,7 @@ cc_feature(
         ":target_cpu_flags_args",
         ":c_std_c11_args",
         ":c_std_gnu11_args",
+        ":c_std_c99_args",
         ":default_cxx_compile_flags_args",
         ":dbg_compile_flags_args",
         ":opt_compile_flags_args",

features/native/markers/BUILD

@@ -75,6 +75,12 @@ cc_feature(
     visibility = ["//visibility:public"],
 )
 
+cc_feature(
+    name = "c99",
+    feature_name = "c99",
+    visibility = ["//visibility:public"],
+)
+
 ## QNX only
 cc_feature(
     name = "dependency_file_named_implicitly",

tests/BUILD

@@ -23,6 +23,7 @@ Root test suite definition combining all test categories.
 test_suite(
     name = "language_and_standards_tests",
     tests = [
+        "//language_and_standards:c99_feature_test",
         "//language_and_standards:c_lang_test",
         "//language_and_standards:cpp11_test",
         "//language_and_standards:cpp14_test",

tests/language_and_standards/BUILD

@@ -38,6 +38,17 @@ cc_test(
     copts = ["-std=c99"],
 )
 
+# Test for: the `c99` toolchain feature (as opposed to explicit -std= copts)
+# Verifies: enabling the `c99` feature switches the compiler onto -std=c99;
+#           a negative-array-size typedef gated on __STDC_VERSION__ only
+#           compiles under c99, so it fails to build if the feature didn't
+#           take effect
+cc_test(
+    name = "c99_feature_test",
+    srcs = ["c99_feature_test.c"],
+    features = ["c99"],
+)
+
 # ============================================================================
 # C++ Standard Version Tests
 # ============================================================================

tests/language_and_standards/c99_feature_test.c

@@ -0,0 +1,29 @@
+/********************************************************************************
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information regarding copyright ownership.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Apache License Version 2.0 which is available at
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ ********************************************************************************/
+
+// Test for: the `c99` toolchain feature (enabled via the `features` attribute
+// below, mirroring `--features=c99`). Unlike the other tests in this package,
+// this does not pass `-std=` via copts: it relies entirely on the toolchain
+// feature to select the C standard.
+//
+// GCC accepts C11-only keywords (_Generic, _Noreturn, ...) as an extension
+// under -std=c99 too, so they don't actually depend on the active standard
+// here. Instead this uses the standard-mandated `__STDC_VERSION__` macro in a
+// real constraint: an array with a negative size is an ISO C constraint
+// violation, so the typedef below only compiles when the active standard is
+// exactly C99.
+typedef char standard_must_be_c99[(__STDC_VERSION__ == 199901L) ? 1 : -1];
+
+int main(void) {
+    return sizeof(standard_must_be_c99) - 1;
+}